In most head coil designs, a double coil structure is used. Typically, the outside coil is a volume coil for RF excitation and the inner coil is a surface coil array for signal reception.
In head imaging, two important design criteria for the coil design are homogeneity in RF excitation and sufficient sensitivity in signal reception. These requirements are usually achieved using the combination of a volume coil, such as a TEM or Birdcage coil, and a surface coil array with active decoupling circuits.
It has been shown that placing a shield on the side of a surface coil can offer good decoupling. This is equivalent to the use of a counter rotating current (CRC) coil, which is also efficient in coil decoupling. However, placing a shield on the side of a surface coil does not allow good control in the coil decoupling.